Thermoplastic elastomers are known. They have many of the properties of thermoset elastomers, yet they are processable as thermoplastics. One type of thermoplastic elastomer is a thermoplastic vulcanizate, which may be characterized by finely-divided rubber particles dispersed within a plastic matrix. These rubber particles are crosslinked to promote elasticity.
Thermoplastic vulcanizates may advantageously be prepared by dynamically vulcanizing a rubber with a phenolic resin while the rubber is being mixed with a thermoplastic resin. U.S. Pat. No. 4,311,628 teaches that thermoplastic vulcanizates that include fully-cured rubber can be produced by using phenolic resin cure systems that include a phenolic curing resin and a cure activator such as stannous chloride. According to this patent, from 5 to 20 parts by weight phenolic resin and 0.1 to 10 parts by weight cure activator, per 100 parts by weight rubber, may be used. Likewise, U.S. Pat. No. 5,952,425 teaches the use of halogen donors in an amount from about 0.01 to 10 parts by weight per 100 parts by weight rubber. Conventionally, the stannous chloride is employed in an excess of 1 part by weight per 100 parts by weight rubber in order to achieve technologically useful products that include fully-cured rubber.
Unfortunately, it is believed that the cure activator significantly contributes to the hygroscopicity of thermoplastic vulcanizates. More specifically, it is believed that the chloride ion associated with the cure activator attracts and absorbs atmospheric moisture. This moisture may impact processing, physical properties, end-product performance, and surface appearance. As a result, moisture pickup control is very important for successful processing. Also, it is often necessary to effectively control the moisture in order to eliminate porosity and obtain good surface finishes and high quality finished parts. Therefore, thermoplastic vulcanizates are typically dried prior to processing. It has also been found that stannous chloride deleteriously impacts the shelf life of thermoplastic vulcanizates as evidenced by the porosity of extrudates that are formed after extended storage of the composition. This porosity is believed to be due to excessive moisture pickup that cannot be alleviated by normal drying procedures, which typically includes about 4 hours at 71° C. within a dryer.
Attempts have been made to reduce the propensity of these thermoplastic vulcanizates to attract and absorb atmospheric moisture. Unfortunately, these attempts have not met with a great degree of success. Where a cure activator such as stannous chloride is completely removed from the system, the ability to fully cure the rubber is precluded as suggested in U.S. Pat. No. 6,143,828. As a result, thermoplastic vulcanizates that exhibit the advantageous properties associated with fully-cured rubber cannot be attained. U.S. Pat. No. 5,952,425 also suggests that cure kinetics are not technologically useful in the absence of stannous chloride. Other attempts to alleviate the problem have included the use of alternate activator systems. For example, U.S. Pat. No. 6,437,030 teaches the use of reduced-halide systems based on metal carboxylates. Unfortunately, these systems tend to have slower cure kinetics and often result in lower cure states. Also, these systems often require preventative measures to compensate for drastic reductions in heat stability that have been observed. As for shelf life, current attempts to minimize the problem have focused on the packaging of the thermoplastic vulcanizates, where special foil liners are often employed. This is not only costly, but also it is not a solution to the problem.
In addition to problems associated with hygroscopicity, thermoplastic vulcanizates prepared with phenolic resin cure systems with a stannous chloride cure activator have exhibited a propensity to plate out, which refers to residue buildup within extrusion dies that can eventually affect engineering tolerances. Therefore, conventional extrusion practices often require routine cleaning of the dies. The prior art has not offered any explanation as to why this plate out occurs and, moreover, has not offered any solution to the problem.
Because phenolic resin cure systems provide significant advantages over other cure systems that may be employed in the production of thermoplastic vulcanizates, there is a continued need to alleviate some of the problems associated with the use of these cure systems in the production of thermoplastic vulcanizates.